Foundations and Applications

Petroleum

Photo by: aberenyi

Petroleum is a naturally occurring complex mixture made up predominantly
of carbon and hydrogen compounds, but also frequently containing
significant amounts of nitrogen, sulfur, and oxygen together with smaller
amounts of nickel, vanadium, and other elements. Solid petroleum is called
asphalt; liquid, crude oil; and gas, natural gas. Its source is
biological. Organic matter buried in an oxygen-deficient environment and
subject to elevated temperature and pressure for millions of years
generates petroleum as an
intermediate
in the transformation that ultimately leads to methane and graphite. The
first successful drilled oil well came in 1859 in Pennsylvania. This is
considered to be the beginning of the modern oil industry. Continuous
distillation of crude oil began in Russia in 1875.

Occurrence

Oil is the largest segment of our energy raw materials use, being 40
percent, while coal use accounts for 27 percent, gas 21 percent, and
hydroelectric/nuclear 12 percent. Although there are 20,000 petroleum
fields known worldwide, more than half of the known reserves are contained
in the 51 largest fields. The Middle East has 66 percent of the known
world reserves. The United States has only 2 percent of the known world
reserves. Hence the need for imports. The Organization of Petroleum
Exporting Countries (OPEC) is important to the international trade and
distribution of this crude oil. There is a growing dependence of the
United States on imports. Although U.S. domestic production has not grown
since the 1950s, imports have grown dramatically, from 0.3 billion barrels
of oil in 1955 to 3.0 billion barrels in 1997. The United States has
increased its percentage of imports, from approximately 13 percent in 1970
to 55 percent in 2000. It uses approximately 18 million barrels of oil per
day. Worldwide production is about 56 million barrels per day. With known
reserves, this level of worldwide production could remain constant for
only 43 years. But there are large volumes of unconventional petroleum
reserves, such as heavy oil, tar sands, and oil shale. These are located
in the Western Hemisphere. Improvements in recovery methods must be made,
and the cost of production must decrease, for these sources to become more
important providers of energy.

The world's first oil well, near Titusville, Pennsylvania,
1863.

Composition

Crude oils vary dramatically in color, odor, and flow properties. There
are light and heavy crude oils; they are sweet or sour (i.e., have high or
low sulfur content, with an average of 0.65%). Several thousand compounds
are present in petroleum. The number of carbon atoms in these compounds
can vary from one to over a hundred. Few are separated as pure substances.
Many of the demands for petroleum can be served by certain fractions
obtained from the distillation of crude oil. Typical distillation
fractions and their uses are given in Table 1. The complexity of the
molecules, their molecular weights, and their carbon numbers increase with
the boiling point. The higher-boiling fractions are usually distilled in
vacuo at temperatures lower than their atmospheric boiling points to avoid
excessive decomposition to tars.

Diesel and heating fuel. Catalytically cracked to naphtha and
steam-cracked to alkenes.

>350°C

Lubricating oil

Lubrication. May be catalytically cracked to lighter fractions.

>350°C

Heavy fuel oil

Boiler fuel. May be catalytically cracked to lighter fractions.

Asphalt

Paving, coating, and structural uses.

naphthenes; and 20 aromatic compounds (such as benzene, toluene, and
xylene). Examples of compounds found or used in petroleum and mentioned in
this article are given in Figure 1.

When any fraction of petroleum is used as a source of energy and burned to
CO
2
and H
2
O, the sulfur is converted into SO
2
in the air. The SO
2
is a major air contaminant, especially in larger cities. With air
moisture it can form H
2
SO
4
and H
2
SO
3
. Much of the sulfur-containing material must be taken out of petroleum
before it can be used as fuel. The current maximum percentage allowable in
gasoline is 0.10 percent S.

Octane Number

One cannot talk about the chemistry of gasoline without understanding
octane numbers. When gasoline is burned in an internal
combustion
engine to CO
2
and H
2
O, there is a tendency for many gasoline mixtures to burn unevenly. Such
nonconstant and unsmooth combustion creates a "knocking"
noise in the engine. Knocking signifies that the engine is not running as
efficiently as it could. It has been found that certain hydrocarbons burn
more smoothly than others in a gasoline mixture. In 1927 a scale that
attempted to define the "antiknock" properties of gasolines
was created. At that time, 2,2,4-trimethylpentane (commonly called
"isooctane") was the hydrocarbon that, when burned pure in
an engine, gave the best antiknock properties (caused the least knocking).
This compound was assigned the number 100, meaning it was the best
hydrocarbon to use. The worst hydrocarbon researchers could find in
gasoline (which when burned pure gave the most knocking) was
n
-heptane, assigned the number 0. When isooctane and heptane were mixed,
they gave different amounts of knocking depending on their ratio: The
higher the percentage of isooctane in the mixture, the lower was the
amount of knocking. Gasoline mixtures obtained from petroleum were burned
for comparison. If a certain gasoline has the same amount of knocking as a
90 percent isooctane, 10 percent heptane (by volume) mixture, we now say
that its "octane number" is 90. Hence, the octane number of
a gasoline is the percent isooctane in an isooctane-heptane

Figure 1. Some compounds found or used in petroleum.

mixture that gives the same amount of knocking as the gasoline being
measured. Thus, a high octane number means a low amount of knocking.

Presently there are two octane scales, a research octane number (RON) and
a motor octane number (MON). RON values reflect performance at 600 rpm,
148.8°C (125°F), and low speed. MON is a performance index
of driving with 900 rpm, 51°C (300°F), and high speed. The
station pumps now give the (R + M)/2 value. Regular is usually 87 to 89
and premium about 92 on this scale.

Certain rules have been developed for predicting the octane number of
different types of gasoline, depending on the ratio of different types of
hydrocarbons in the mixtures:

The octane number increases as the amount of branching or the number of
rings increases.

The octane number increases as the number of double and triple bonds
increases.

Additives

In 1922 two chemists working at General Motors, Midgley and Boyd, were
looking at different substances that would aid the combustion of gasoline
and help the knocking problems of engines. In other words, they were
seeking methods of increasing the octane rating of gasoline without
altering the

An oil refinery at Cap Bon, Tunisia.

hydrocarbon makeup. They were also interested in cleaning up the exhaust
of automobiles by eliminating pollutants such as unburned hydrocarbons and
carbon monoxide through more complete combustion. By far the best
substance that they found was tetraethyllead. Lead in this form aids in
breaking carbon-carbon and carbon-
hydrogen bonds
. But the lead oxide formed in the combustion is not
volatile
and would accumulate in the engine if dibromoethane and dichloroethane
were not added. In the environment the lead dihalide formed undergoes
reaction by sunlight to elemental lead and
halogen
, both of which are serious pollutants.

For the past several years other additives have been tried. Ethyl alcohol
has become popular. When 10 percent ethyl alcohol is mixed with gasoline
it is called gasohol and it is popular in states with good corn crops, as
the alcohol can be made from corn fermentation. An attractive alternative
to tetraethyllead is now methyl
t
-butyl ether (MTBE). MTBE has been approved at the 7 percent level since
1979. From 1984 to 1995 its production grew by 25 percent per year, the
largest increase of any of the top chemicals. The Clean Air Act of 1991
specifies that the gasoline must be at the 2.0 percent oxygen level. Thus,
MTBE, ethyl
t
-butyl ether (ETBE), ethanol, methanol, and other ethers and alcohols had
to be added to gasoline at higher levels. The product is called
reformulated gasoline (RFG), and it may cut carbon monoxide levels and may
help to alleviate ozone depletion. But improved

Figure 2. Petroleum refining processes.

emission control systems may make this high-level input unnecessary.
Currently MTBE accounts for 85 percent of the additive market, with 7
percent being ethanol and the remaining 8 percent split by other
chemicals. In 1999 California took steps toward banning MTBE. In 2000 some
factions called for a U.S. ban on MTBE and for increased use of ethanol to
meet the oxygenate requirement. MTBE has been found in drinking water. But
ethanol cannot be blended into gasoline at the refinery because it is
hygroscopic and picks up traces of water in pipelines and storage tanks.
Also, ethanol shipped away from the Midwest, where it is made by corn
fermentation, would add to the cost of gasoline. Gasohol may increase air
pollution because gasoline containing ethanol evaporates more quickly.
Studies and debate continue.

Refinery Processes

There are processes that are used to refine petroleum into useful
products. These are important processes for the gasoline fraction because
they increase the octane rating. Some of these processes are used to
increase the percentage of crude oil that can be used for gasoline. They
were developed in the 1930s when the need for gasoline became great with
the growing automobile industry. These processes are also keys in the
production of organic chemicals. An example of each of these processes is
given in Figure 2. One process is cracking. In
catalytic
cracking, as the name implies, petroleum fractions of higher molecular
weight than gasoline can be heated with a
catalyst
and cracked into smaller molecules. This material can then be blended
into the refinery gasoline feed.

Catalytic reforming leaves the number of carbon atoms in the
feedstock
molecules usually unchanged, but the resultant mixture contains a higher
number of double bonds and aromatic rings. Reforming has become the
principal process for upgrading gasoline. High temperatures with typical
catalysts of platinum and/or rhenium on alumina and short contact times
are used. A typical example is the reforming of dimethylcyclopentane to
toluene. Straight-run gasoline can be reformed to as high as 40 to 50
percent aromatic hydrocarbons, of which 15 to 20 percent is toluene.

Although cracking and reforming are by far the most important refinery
processes, especially for the production of petrochemicals, two other
processes deserve mention. In alkylation, alkanes (hydrocarbons with no
double or triple bonds) react with alkenes (hydrocarbons with double
bonds) in the presence of an acid catalyst to give highly branched
alkanes. In polymerization an alkene can react with another alkene to
generate dimers, trimers, and tetramers of the alkene. As an example,
isobutylene (C
4
) reacts to give a highly branched C
8
alkene dimer.

Natural Gas

Natural gas can be as high as 97 percent methane, the remainder being
hydrogen, ethane, propane, butane, nitrogen, hydrogen sulfide, and heavier
hydrocarbons. A typical mixture contains 85 percent methane, 9 percent
ethane, 3 percent propane, 1 percent butanes, and 1 percent nitrogen. Uses
of natural gas by all industry include fuel (72%) and the manufacture of:
inorganic chemicals including ammonia (15%), organic chemicals (12%), and
carbon black (1%). The ethane and propane are converted to ethylene and
propylene. The methane is purified and used to make a number of other
chemicals.